1. Field of the Invention
This invention relates to a diagnostic X-ray and, more particularly, to a diagnostic X-ray apparatus utilizing a digital X-ray subtraction imaging technique.
2. The Prior Art
A digital X-ray subtraction imaging technique using an imaging intensifier-TV chain is described in U.S. Pat. Nos. 4,204,225 and 4,204,226. Each of these patents discloses a real-time digital X-ray subtraction imaging method and apparatus used in digital radiography or digital fluoroscopy technology. These digital X-ray subtraction imaging apparatus include an image intensifier, a TV camera whose output is converted into digital format, and an image processor incorporating two digital memories.
More specifically, U.S. Pat. No. 4,204,225 discloses that in carrying out the mask mode of this technology, a first memory system is preferably employed to integrate and store digital mask video signals by digitization of video signals from an image intensifier-TV chain during an initial mask time interval utilizing a relatively large number of television fields. A second memory system integrates ongoing digital video signals and provides an output of these integrated video signals from which the mask video signals delivered by the first memory system are subtracted. The resulting digital difference video signals are supplied to a digital-to-analog converter which provides corresponding analog difference video signals for display by a display device. Alternatively, the analog signals may be fed to a multiformat camera for making a selected number of radiogrphic exposures on a single film.
In the time interval difference mode disclosed in U.S. Pat. No. 4,204,226, a series of difference images is produced by integrating digital video signals over a series of successive time intervals, performing a series of subtractions between the sets of successive integrated video signals stored in the memories to produce a series of digital difference video signals, and then converting such digital difference video into visibly displayed difference images representing changes in the X-ray image during successive time intervals.
One advantage of digital radiography apparatus is its capability to perform angiography by means of intravenous injections instead of by intra-arterial catheter techniques with their higher risks. Still another advantage of digital radiography apparatus is its capability to provide improved low contrast detection, namely, to amplify subtle amounts of contrast media in arteries, leading to results better than those normally achievable by film methods. Another advantage of digital radiography apparatus employing pulsed X-rays is that significant loss of spatial resolution due to physiological motion can be prevented because the short radiation pulse for each image results in less loss of detail during fluoroscopy of moving objects.
In such digital radiography apparatus, however, the TV camera is read out during the exposure and then a predetermined number of frames are averaged to produce an image. At least one frame, and generally several frames, are required to allow the video signal level to stabilize. The start of the camera readout coincides with initiation of the pulsed X-ray exposure so that motion blurring of the integrated X-ray image is a potential problem if the patient moves during image acquisition. This effect is further aggravated by the charge time or log of the TV pickup tube.
Therefore, the readout TV frame of such an apparatus includes charges remaining on the target due to preceding pulsed X-ray exposures. Furthermore, the levels of contrast to be detected are extremely low due to using a low kVp X-ray beam for each pulsed exposure, and a high signal-to-noise ratio is required to obtain meaningful diagnostic data.